In rock bits of the type commonly used in drilling oil and gas wells, for example, rock cutter cones are mounted on pins on the bit for pulverizing rock. High pressures are applied to such a rock bit and there are extreme loads on the bearing surfaces between the cone and leg. In some such rock bits the cones are mounted on the leg journal by a combination of anti-friction bearings using balls and rollers. In such an arrangement the space required for the rollers reduces the thickness of the cone and/or leg so that breakage sometimes occurs.
An alternative cone bearing structure particularly useful for extreme load bearing conditions has a combination of what are often considered friction and anti-friction bearings. A friction bearing indicates a structure where one material slides on another, usually with lubrication. These are referred to herein as solid journal bearings. Anti-friction bearings refer to bearings with rolling contact such as balls or rollers. In such a cone structure, the brunt of the radial load on the bearing is carried by the solid journal portion and thrust loads are partly accommodated by ball bearings that also serve to lock the cone on the leg bearing. A variety of approaches have been taken to the solid journal bearings to obtain prolonged life. The most usual approach is to provide uniformly hard bearing surfaces of high hardness. Typically the hardness is of the order of about Rockwell C60. Bearing surfaces of extremely hard material such as tungsten carbide have also been proposed.
In another embodiment the bearing surface in the steel body of the cutter cone is provided with a layer of aluminum bronze around the full periphery. Such an embodiment has extremely long average life under the severe conditions encountered by rock bits.
Another approach is proposed in U.S. Pat. No. 3,235,316 issued Feb. 15, 1966, to J. R. Whanger. In accordance with that patent, the bearing surface in the cutter cone is hardened carburized steel with intermittent areas of soft anti-galling material such as certain silver alloys. The anti-galling material has a hardness very much lower than that of the carburized steel.
Tests have been made of a cutter cone having a solid journal bearing surface in the cone formed entirely of a hard, wear resistant material welded in place on the steel body like that "hard facing" commonly used on various areas of well drilling tools. When successful, this bearing surface with a hard facing was found to give life comparable to that of aluminum bronze bearing surfaces when the bearing was lubricated in the normal manner. It was found to be significantly superior to bearings with aluminum bronze if the grease seal on the bearing failed, and the bearing was subjected to the types of abrasive materials present in well drilling operations. Hard facing has been applied uniformly to rock bit friction pins (see Metals Handbook, Welding and Brazing, Vol. 6, American Society for Metals, (1971) page 588).
Although the tests indicated superior performance by cone bearings having a surface of hard facing material, manufacture of satisfactory rock cutter cones with a hard facing on the bearing surface proved to be difficult, and there was an unacceptably high rejection rate. A reason for this lies in the almost zero ductility of the hard facing material and its susceptibility to cracking due to differential thermal expansion during the heat treating operations on the cutter cone that must follow its deposition. It was not uncommon to form substantial cracks in the hard facing material which could propagate through the hardened steel cutter cone if the bit were put into service. Breakage of a cutter cone in service can cause significant delay in drilling operations. Attempts to use hard facing materials with low ductility in solid journal bearings of cutter cones have not been completely satisfactory.
In well drilling operations the life of the rock bit is of considerable importance. A substantial time may be involved in round tripping a drill string to change the bit. This is particularly true in deep holes where the time required to pull a drill string is rather long. It is, therefore, highly desirable to prolong the life of rock bits so that more rock can be drilled before it is necessary to change the bit. The bearings employed on such rock bits should have a substantial lifetime so that they do not fail before the bit becomes dulled and must otherwise be replaced. Even moderate increases in lifetime can amount to substantial cost savings.